In a medium bowl, whisk together the egg yolks and ¼ cup of granulated sugar until creamy and pale. In a saucepan, heat the whole milk over medium heat until it reaches 165°F.

Gradually pour the hot milk into the egg mixture while whisking continuously to prevent scrambling, then return to low heat until thickened, about 5 minutes.

Transfer the custard base to a large mixing bowl. Add the heavy cream, vanilla extract, and a pinch of salt, stirring gently until combined.

Cover the bowl with plastic wrap and refrigerate for at least 4 hours, preferably overnight, to chill thoroughly.

Chop the dark chocolate into small pieces and melt in a small saucepan over medium-low heat, stirring often until smooth.

Once melted, remove from heat, add a teaspoon of butter, and transfer chocolate to a piping bag or resealable plastic bag.

Pour the chilled custard into an ice cream maker and churn according to the manufacturer's instructions for about 25-30 minutes.

In the final minutes of churning, drizzle the melted chocolate into the ice cream to create ribbons.

Transfer the ice cream to an airtight container and freeze for several hours, preferably overnight, to firm up.